Index: Modules/mathmodule.c
===================================================================
--- Modules/mathmodule.c	(revision 81964)
+++ Modules/mathmodule.c	(working copy)
@@ -453,8 +453,8 @@
 static double
 m_erfc_contfrac(double x)
 {
-    double x2, a, da, p, p_last, q, q_last, b;
-    int i;
+    double x2, a, da, p, p_last, q, q_last, b, result;
+    int i, saved_errno;
 
     if (x >= ERFC_CONTFRAC_CUTOFF)
         return 0.0;
@@ -472,7 +472,12 @@
         temp = p; p = b*p - a*p_last; p_last = temp;
         temp = q; q = b*q - a*q_last; q_last = temp;
     }
-    return p / q * x * exp(-x2) / sqrtpi;
+    /* Issue #8986: On some platforms, exp sets errno on underflow to zero;
+       save the current errno value so that we can restore it later. */
+    saved_errno = errno;
+    result = p / q * x * exp(-x2) / sqrtpi;
+    errno = saved_errno;
+    return result;
 }
 
 /* Error function erf(x), for general x */